1. Field of the Invention
The present invention relates to a current source circuit using current mirror circuits.
2. Description of the Prior Art
Today a current source circuit using current mirror circuits is proposed. FIG. 1 is the circuit diagram of a current source circuit using current mirror circuits. In FIG. 1, a current source circuit 1 is comprised of an operational amplifier 2, an N-channel MOSFET (hereinafter simply called a "MOS transistor") Q1, P-channel MOSFETs (hereinafter simply called a "MOS transistor") Q2 and Q3 that compose a current mirror circuit and a resistor R1.
A reference signal (Vref), which is described later, is supplied to the non-inversion input (positive input) of the operational amplifier 2, and a feedback signal is applied to the inversion input (negative input). The feedback signal applied to the inversion input (negative input) is a voltage value at point A shown in FIG. 1, and which is a potential of the connection point between the MOS transistor Q1 and the resistor R1. The output of the operational amplifier 2 is supplied to the gate of the MOS transistor Q1 and the output turns the MOS transistor Q1 on/off.
The MOS transistors Q2 and Q3, which compose the current mirror circuit, have the same characteristics and the same mirror current flows in the MOS transistors Q2 and Q3. For example, when a gate voltage is applied to the gate of the MOS transistor Q1 from the operational amplifier 2, the MOS transistor Q1 is turned on, a current flows in the MOS transistor Q2. Simultaneously, an output current (mirror current) Iout with the same current value flows in the MOS transistor Q3.
The potential at point A is the potential of the reference signal (Vref) of the operational amplifier 2. Therefore, while the MOS transistor Q1 is turned on, the voltage applied to the resistor R1 is Vref and the current (Iref) that flows in the resistor R1 is the voltage of the reference signal (Vref) divided by the resistance value of the resistor R1. This current Iref flows in one direction in the current mirror circuit, and the output current (mirror current) (Iout) shown in FIG. 1 is the same as the current (Iref).
Therefore, when in the configuration it is assumed that the reference signal varies, the output current (mirror current) also varies in the same way. For example, when a triangular wave is used for the reference signal (Vref), the output current (mirror current) Iout becomes a triangular wave.
In this way, according to the current source circuit shown in FIG. 1, the output current varies as the reference signal (Vref) varies and the desired output current can be obtained. However, in the current source circuit, the response speed is slow, which is a problem. This is because the operational amplifier 2 is used and the feedback circuit is used at the same time. Specifically, many transistor circuits are used in the operational amplifiers and it takes much time to drive the circuit. The use of the feedback circuit requires a period of time to return the signal.
FIG. 2 shows that the output current (mirror current) Iout delays from the reference signal (Vref) In FIG. 2, a waveform represented by Vref is the reference signal inputted to the operational amplifier 2, and a dotted waveform represented by Iout indicates the output timing of the output current (mirror current) The output current (mirror current) Iout delays from the reference signal (Vref), and a time lag of time T is generated between the reference signal (Vref) and the output current (mirror circuit) Iout.
This time lag is a problem when the output current (mirror current) Iout is actually used. For example, when the output current (mirror current) Iout is used as an oscillator modulation, the modulation timing is delayed. When a pulse signal is generated using the output current (mirror current) Iout, the pulse signal with a targeted timing cannot be generated due to the delay of the output current (mirror current).